DESCRIPTION: The long-term goal is to understand and quantify the processing of temporally-complex sounds in the mammalian auditory system. The Specific Aims for this cycle address neural mechanisms of "sound source identification" (Yost, 1991). These mechanisms are likely to make use of temporal information in the stimulus. Responses of single neurons in the inferior colliculus (IC) of the chinchilla will be measured. Studies of the representation of complex tones that are heard as if they consist of two separate sound sources are proposed. These studies take advantage of a novel technique for evaluating the processing "weight" that IC neurons give to particular spectral components. Other studies will examine neural correlates of the masked threshold, for stimulus configurations that promote "across-channel" processing. The psychoacoustic phenomena called "Comodulation Masking Release" (CMR) and "Modulation Detection Interference" (MDI) will be used as a framework to study integrative neural mechanisms that contribute to the identification of sound sources. In CMR, across-channel processing makes the detection of a signal in noise easier. In MDI, across-channel processing makes the detection of modulation more difficult. The neural mechanisms that underlie CMR and MDI are not known, but they must involve integration of information across bandwidth. The IC is likely to participate in that integration. Experiments to quantify the ability of IC neurons to represent temporal envelopes will also be conducted, to aid in the interpretation of other experiments. Specific Aim 1 tests the hypothesis that IC neurons respond differentially to components in a complex sound that are perceived as belonging to separate sound sources because they differ in harmonicity. Specific Aim 2 tests the hypothesis that the thresholds of IC neurons to tones in comodulated noise bands are lower than their thresholds in noise bands with deviant envelopes; that is, that correlates of CMR will be found in the IC. Specific Aim 3 tests the hypothesis that rsponses of IC neurons to modulated tones are resistant to interference from other modulated tones; that is, that correlates of MDI will not be found in the IC. Specific Aim 4 is to measure the perception of mistuning and CMR in the chinchilla with psychophysical techniques.